The use of the cardiac catheter as a tool for laboratory and clinical investigation has opened a new era in the understanding of many physiologic and clinical aspects of cardiovascular function and disease. This has been particularly true in the field of congenital heart disease. There have been a few attempts to perform therapeutic procedures using cardiac catheterization. Our technique to produce a palliative atrial septostomy using balloon-tipped catheter has been one of the most productive areas of this type of study. We have also had a small clinical experience in transcatheter closure of atrial septal defects, and in transcatheter closure of patent ductus arteriosus. We propose to continue our experiments on: 1) using the catheter as a means of introducing detachable prostheses for closure of septal defects and patent ductus arteriosus, 2) developing self-seating prostheses for rapid defect closure during cardiac surgery, 3) development of a catheter-knife.